Meh!
I think it starts from an interesting idea: assault instead of defend. However, I think there is absolutely no incentive to do anything else but an infantry rush. That makes the game very unidimensional and repetitive.
The art is nice, and I like to see the soldiers fall as they approach the castle: that's well-done.
Again though, I don't see any incentive to go beyond level 1: it's the same.
In terms of suggestions, I would say that it would be great if there was some progression other than being able to send 50% more infantry at the castle. Make it impossible for us not to use units other than infantry or archers. You could make it so that these units are either worth the price you pay for them (which right now they aren't: 20 infantry=1 cannon, not in this game...), or that they have abilities that are required for victory.
Overall it is a very good idea, but it isn't well-executed.